Table Of Content**VolumeTitle**
ASPConferenceSeries,Vol.**VolumeNumber**
**Author**
(cid:13)c**CopyrightYear**AstronomicalSocietyofthePacific
VAST-a real-timepipeline fordetecting radio transients and
variablesonthe AustralianSKA Pathfinder (ASKAP)telescope
2
1
0 JayBanyer1,2,TaraMurphy1,2,3,andtheVASTCollaboration
2
1Sydney Institute forAstronomy(SIfA),SchoolofPhysics,TheUniversity of
n
Sydney, NSW2006,Australia
a
J
2ARCCentreofExcellence forAll-skyAstrophysics (CAASTRO),The
5
University ofSydney, NSW2006,Australia
1
3SchoolofInformation Technologies, TheUniversity ofSydney, NSW2006
]
M Australia
I
. Abstract. TheAustralianSKAPathfinder(ASKAP)isanextgenerationradiotele-
h
scope currently under construction in Western Australia. The fast survey speed and
p
widefieldofviewmakeitanidealinstrumentforblindtransientssearches.TheASKAP
-
o Variable and Slow Transients (VAST) survey is a one of the major science programs
r planned for ASKAP. The scientific goals of VAST include the detection and charac-
t
s terisationofawiderangeoftransientandvariablephenomena,fromgamma-rayburst
a afterglowstoextremescatteringevents,ontimescalesof5secondsorlonger.
[
We describe the data and processing challenges involved in running the VAST
1 real-timetransientdetectionpipeline.ASKAPwillproduce2.5GBofvisibilitydataper
v second,transformedintoone8GBimagecubeevery5seconds.Eachcubewillcontain
0 approximately twenty 100 megapixel images with 100s of radio sources detected in
3 eachepoch. TheVASTpipelinewillmeasureandmonitorallofthesesources,detect
1 variablesandtransientsandgeneratealertsusingtheVOEventframework.
3
ThegoaloftheVASTDesignStudyistodevelopaprototypepipelinetoestablish
.
1 anddemonstratethe functionalityof thefinalASKAP pipeline. We givean overview
0 oftheprototypepipeline’sfunctionality,technicalimplementationandcurrentstatus.
2
1
:
v
i
X
1. TheAustralian SKAPathfinder(ASKAP)
r
a
The Australian SKA Pathfinder (ASKAP, Johnston etal. 2008) is a powerful radio
telescope currently under construction inWestern Australia. Itisa36dish radio inter-
ferometer with exceptional survey speed: it can image the entire southern sky to high
sensitivity intwonights. Thefastsurveyspeedandwidefieldofviewmakeitanideal
instrument forblindtransients searches.
ASKAP is located at the Murchison Radio-astronomy Observatory (MRO), ap-
proximately 315km north east of Geraldton, Western Australia. The MRO is also the
location of the Murchison Widefield Array (MWA) telescope and is the candidate site
forAustraliaandNewZealand’sbidtohosttheSquareKilometreArray(SKA).
A 6 antenna array called BETA is currently being installed and will be commis-
sioned during 2012. ASKAP is scheduled to begin scientific observations with a 6 to
12antennaarrayduring2013.
1
2 JayBanyer,TaraMurphy,andtheVASTCollaboration
2. TheVASTSurvey
ASKAP will give us an unprecedented opportunity to investigate the transient sky at
radio wavelengths. Its wide-field survey capabilities will enable the discovery and in-
vestigation of variable and transient phenomena from the local to the cosmological,
including flare stars, intermittent pulsars, X-ray binaries, magnetars, extreme scat-
tering events, intra-day variables, radio supernovae and orphan afterglows of gamma
ray bursts. In addition ASKAP probes unexplored regions of phase space where new
classesoftransientsources maybedetected.
The VAST survey (Murphy&Chatterjee 2009) is one of ten major science pro-
grams planned for ASKAP. It will involve several survey regimes including one that
images most of the visible sky every night for two years. This will produce measure-
mentsofonemillionsourcesoverhundreds ofepochs.
3. TransientPipelineFunctionality
The VAST transient pipeline will detect transients and variables by analysing the im-
ages produced by the ASKAP imaging pipeline. The VAST pipeline operates by ex-
tracting and measuring sources in images, constructing and analysing light curves and
generating notifications of detections. Thisapproach isbased on techniques described
byBannister etal.(2011). Thefunctionality oftheVASTpipeline isillustrated inFig-
ure1anddescribed inTable1.
TheVASTcollaboration isalsoinvestigating thetechniques oftransient detection
usingimagesubtraction andvisibility domaintransient detection (Trottetal.2011).
Figure1. IllustrationofVASTpipelinefunctionality
4. CapacityChallenges
Thetraditionalmodelforradioastronomyistotakethedatafromthetelescopeandpro-
cess itoff-site some timelater. ASKAPproduces too much data to store and transport
VAST-detectingradiotransientsandvariableswithASKAP 3
Table1. DescriptionofVASTpipelinefunctionality
Function Description
Sourcefinding Blind source finding is performed on every image. This
produces a list of detected sources with position, flux
(brightness) andsize.
Sourceassociation Eachdetectedsourceisassociatedwithonefromthemas-
terlistbyfindingthemastersourceatthesamepositionas
thedetection. Ifnomastersourceisfoundatthatposition
thenanewmastersourceisadded.
Imagequalitycontrol The image is assessed for quality by comparing the mea-
sured parameters of the detected sources to a list of
well-characterised sourcesusingatechniquedescribed by
Bannister etal.(2011).
Sourcemonitoring Master sources known to be in the field that were not de-
tected by thesource finder are measured byattempting to
fitaGaussianatthepositionwherethesourceisexpected
to be. If no source can be fit then an upper-limit on the
fluxatthatposition istakeninstead.
Lightcurvecreation All measurements for each source are collated into a ra-
dio light curve -a collection of flux measurements for all
epochs wherethesourcewasobserved.
Detect and classify tran- Theupdatedlightcurvesarescannedtodetecttransientor
sientsandvariables variable behaviour. Sources that exhibit transient or vari-
able behaviour areclassified using amachine-learning al-
gorithm. Thisattemptstodeterminewhatkindofphysical
phenomenon iscausing thevariability.
Notification Detectionsandclassificationsaresubjectedtoqualitycon-
trolandcross-matching toexternalcatalogues. Thosethat
appeargenuineareaddedtoanarchiveandthecommunity
isalertedusingVOEventnotifications.
sotheVASTtransient surveywillbeimplementedasanearreal-timepipeline running
onthesamelargecomputingcluster astheASKAPimagingsystems.
ASKAP’s imaging pipeline will produce two streams of images: one with inte-
gration times of 5 seconds, the other with longer integration times depending on the
observing mode e.g. 1 minute, 1 hour, 8 hours. Each 5 second image cube will con-
tain approximately twenty ˜100 megapixel images: ˜5 frequency bands each with 4
polarisations. Eachcubewillbe˜8GBinsize.
In the transient pipeline the 5 second image stream will require approximately
12,000 source measurements per second (approximately 100 sources per square de-
gree, an image size of 30 square degrees, 20 images per cube, one cube every 5 sec-
onds). Light curves for approximately 3,000 sources will change each second. Each
source detection requires aconesearch toassociate thedetection withaknown source
4 JayBanyer,TaraMurphy,andtheVASTCollaboration
position. 3,000 cone searches will be performed each second however most of these
can be optimised by searching a cached list of the known sources in the current field.
The cached list will be refreshed each time the telescope moves to a new field, which
willhappennomorefrequently thaneveryminuteorso.
The image stream with longer integrations (e.g. 1 hour) will produce images at
a lower rate but each image will contain more sources due to higher sensitivity. The
sourcemeasurementloadforthisstreamwillvarydepending ontheintegration time.
5. VASTPipelinePrototype
A prototype transient pipeline has been developed as part of the VAST Design Study.
It allows iterative development of the functional requirements of the final ASKAP
pipeline. The prototype pipeline will also be used to perform transient detection on
data from other telescopes, including: the Murchison Widefield Array (MWA), the
Australia Telescope Compact Array (ATCA), the Very Large Array (VLA), and the
SKAMolongloPrototype(SKAMP).
TheprototypetakesaseriesofFITSimagesasinput,detectsandmeasuressources,
performsimagequalitycontrol,constructslightcurvesandoutputsresultstoadatabase.
Transientdetection, classification andVOalertsarebeingdeveloped.
The prototype uses a custom source finder that is able to characterise multiple
overlapping sources moreaccurately than other existing source finders (Hancock etal.
inprep).
Adynamicwebsiteprovidesauserinterfacetothedatabase. Itspurposeistohelp
visualise anddiagnose thepipeline output easily andefficiently. Itcanbrowsethelists
of images, master sources and detections, plot light curves, create postage stamps and
perform cross-matching withexternalcatalogues.
The prototype has already proven to be a useful tool for analysing datasets from
radio telescopes even when not looking for transients due to its ability to measure and
reportonallsources inallimageswithverylittleeffort. Thisassistswithchecking the
imagequality, calibration andpositional accuracyofthedataset.
5.1. Languages,ToolsandLibraries
The prototype pipeline is developed using the Python programming language and the
Django web framework. These were chosen for their suitability for rapidly changing
requirements during the VAST Design Study and the availability of astronomical li-
braries. PostgreSQL is used for the SQL database with the Q3C plugin for optimised
celestial coordinate searches (conesearches).
References
Bannister,K. W., Murphy,T., Gaensler,B. M., Hunstead,R.W., &Chatterjee, S.2011,MN-
RAS,412,634
Johnston,S.,etal.2008,ExperimentalAstronomy,22,151
Murphy, T., & Chatterjee, S. 2009, VAST Memo 1, Tech. Rep. 1. URL
http://www.physics.usyd.edu.au/sifa/vast/uploads/Main/vast_memo1.pdf
Trott,C.M.,Wayth,R.B.,Macquart,J.-P.R.,&Tingay,S.J.2011,ApJ,731,81
